THE SPECTRAL DEPENDENCE OF ELECTRON CENTRAL-AXIS DEPTH-DOSE CURVES

Electron linac fields are usually characterized by the central-axis pr actical range in water, R(p), and the depth of half maximum dose, R(50 ), for dosimetry, quality assurance, and treatment planning. The quant itative relations between the range parameters and the intrinsic linac beam's energy structure are critically reviewed. The spectral quantit y [E(0)] is introduced which is defined as the mean energy of the inc ident spectral peak, termed the ''peak mean energy.'' An analytical mo del is constructed to demonstrate the predicted relation between polye nergetic spectral shapes and the resulting depth-dose curves. The mode l shows that, in the absence of electrons at the patient plane with en ergies outside about [E(0)]+/-0.1[E(0)]*, R(p) and R(50) are both det ermined by [E(0)]. This analytical approximation is confirmed by a Mo nte Carlo calculation comparing two different idealized incident spect ra. The effect of contaminant lower energy or wide-angle scattered ele ctrons is also discussed. The effect of the width of the intrinsic ene rgy spread on the shape of the depth-dose curve is investigated using Monte Carlo depth-dose simulations based on measured linac energy spec tra having energy spreads (full width at half maximum) as large as 20% . These simulations show that the energy spread has only a small effec t on the shape of the central-axis depth-dose curve.